OpenBSD: Remove incorrect -pthread preprocessor define _POSIX_THREADS and replace

with _REENTRANT. Also remove undef _POSIX_THREADS in phread.h.

Patch by Brad Smith!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@155535 91177308-0d34-0410-b5e6-96231b3b80d8